This invention relates to a method of producing submicron nonagglomerated particles of material and more particularly to a method of producing particles in the range of 0.05 to 0.5 micrometers from a gas or gases.
There is a growing need in the production of ceramics with high strength at high temperatures for starting powders with carefully controlled properties. The ideal powder for such application would consist of spherical non-agglomerated submicron particles with carefully controlled compositions and size distribution. It has been suggested (1) that the particles should be uniformly sized but theoretical arguments suggest that a carefully controlled size distribution with a definite upper bound in size would result in higher green compact densities. In either case, precise control of particle size, morphology, and agglomeration is needed. These properties make it possible to produce very uniformed packing of the starting materials, a critical step in ceramics processing.
A variety of methods are presently used to generate these starting powders. Extreme uniformity of particle size is achievable by solution synthesis, such as described by E. A. Barringer and H. K. Bowen "Formation Packing and Centering of Monosphere TiO2 Powders", Communications of the American Ceramic Society C-199-c-201 December 1982: and J. Gobet and E. Matijevic "Preparation of Uniform Colloidal Cadmium and Lead Selenide Particles", Journal of Colloid and Interface Science 100, 1984. However, this technique is applicable to a limited range of compositions. Highly uniform powders of silicon, silicon carbide and silicon nitride have recently been generated by laser-induced pyrolysis of silane, such as disclosed by W. R. Cannon, S. C. Danforth, J. H. Flint, J. S. Haggerty, and R. A. Marra, "Sinterable Ceramic Powders from Laser-Driven Reactions: I. Process Description and Modeling," Journal of American Ceramic Society 65-7-1982 and W. R. Cannon, S. C. Danforth, J. S. Haggerty, and R. A. Marra "Sinterable Ceramic Powders from Laser-Driven Reactions: II. Powder Characteristics and Process Variables," Journal of American Ceramics Society 65-7-1982.
Thermally induced vapor phase reactions have also been used to produce a variety of powders. Oxide particles are produced by vapor phase oxidation of metal halides in externally heated furnaces, such as disclosed by Y. Suyama and A. Kato, "TiO.sub.2 Produced by Vapor Phase Oxygenolysis of TiCl", Journal of American Ceramic Society 65-7-1982. Oxide particles are also produced in flames, such as disclosed by G. D. Ulrich, "Theory of Particle Formation and Growth in Oxide Synthesis Flames", Combustion Science Technology 4,1971 and G. D. Ulrich, B. A. Milnes and N. S. Subranmanian, "Particle Growth in Flames: II. Experimental Results for Silica Particles," Combustion Science Technology 14-1976 and G. D. Ulrich and N. S. Subranmanian "Particle Growth in Flames: III. Coalescence as a Rate Controlling Process," Combustion Science Technology 17, 1977. Silicon nitride powders have been synthesized in a reaction of ammonia and silane in a heated tube as disclosed by S. Prochazka and C. Greskovich "Synthesis and Characterization of a Pure Silicon Nitride Powder", Ceramic Bulletin 57,1978.
A common feature of these processes is the rapid production of condensible products by gas phase reactions leading to the formation of large numbers of very small particles. The residence time is generally long enough for appreciable growth by coagulation and since this growth takes place at high temperature, for sintering of the agglomerates. This results in low-density flocs that can make subsequent processing of the powder difficult. The Prochazka article on page 579 points out on the top of the second column that the silicon nitride powders prepared were amorphous. At the beginning of the text in the second column it is pointed out that the furnace was heated to a selected temperature and there is no discussion of temperature control, so an isothermal reaction at a constant temperature is implied. Again, on page 580 in the second column under the heading Crystallization Behavior of Amorphous Silicon Nitride it is pointed out that all of the powders prepared were amorphous to x-rays.
Another article by S. K. Friedlander "The Behavior of Constant Rate Aerosol Reactors", Aerosol Science and Technology 1:3-13 (1982), at page 3 points out that the reactor is a constant rate aerosol reactor. The stage diagram on the top of page 5 of this article points out the nucleation transition coagulation and agglomeration stages. Coagulation is further discussed under the title "Coagulation and Surface Area" on page 10.
Two additional articles on laser formation, one by John H. Flint, Robert A. Marra, and John S. Haggerty, "Powder, Temperature, Size and Number Density in Laser-Driven Reactions", Aerosol Science and Technology, 5:249-260 (1986): and a second by J. S. Haggerty, G. Garvey, J. M. Lihrmann and J. E. Ritter, "Processing and Properties of Reaction Bonded Silicon Nitride made from Laser Synthesized Silicon Powders", presented at the MRS Conference, Dec. 2-5, 1985, again point out agglomeration in the Flint article on page 258, column 2. The Haggerty article, however, although referring to aggregates on page 3 thereof, points out that compact densities improve with the elimination of the fines and aggregates which were centrifugally separated to produce a powder with a more uniform particle size distribution though formed during the process.
Another article by H. Komiyama, T. Kanai, and H. Inoue "Preparation of Porous Amorphous and Ultrafine TiO2 Particles by Chemical Vapor Deposition", from Chemistry Letters of the Chemical Society of Japan, 1984, pages 1283-1286, also points out that titanium oxide particles formed were amorphous and porous. The scanning electron micrographs illustrated in FIG. 3 on page 1285 show the size and agglomerate nature. Low temperature chemical vapor deposition was used in this approach and the temperature of the reactor was maintained constant within 10.degree. C. within a zone extending twenty centimeters downstream of the nozzle.
In a related case filed by R. Flagan and M. Alam, Serial Number 572604, filed Jan. 20, 1984 now U.S. Pat. No. 4,642,227, issued Feb. 10, 1987, a continuation in part of application Ser. No. 409941, filed Aug. 20, 1982, entitled "Reactor for Producing Large Particles of Materials from Gases," a method was developed to meet a need for this synthesis of high purity bulk materials using aerosol processes. Previous attempts to produce bulk materials by gas phase reactions in flow reactors generated very fine particles that were not well suited for the anticipated processing methods. In this case a method was presented with which the formation of new particles could be suppressed to allow a small number of seed particles to grow to large size. The growth was predominatedly by chemical vapor deposition. The seed particles were produced by nucleation in a separate reactor stage and then diluted to limit the number concentration to allow the desired growth given the amount of reactant introduced in the gas system. These particles were of a size larger than one micron but less than one hundred microns and the process was used for example to produce extremely pure silicon such as for use in semiconductor devices.
The current invention focuses on a different need. That of supplying fine submicron particles for use in the synthesis of bulk ceramics and other powder-based materials. When a material is synthesized from a powder, voids in the green packing of powder lead to the formation of defects that reduce the strength of the final bulk structure. The size of the defects is directly related to the size of the largest particles in the starting powder. This has led ceramists to define the ideal powder as one consisting of monosized particles in the submicron size range. The particles should also be approximately equiaxed so that the orientation of packing does not promote the formation of large voids. There is reason to believe that higher green packing density would be achieved with a powder that contains two or more sizes of particles, but it is clear that the size distribution should be carefully controlled with strict limits on the maximum particle size and control of particle morphology. The particle should be approximately spherical with a minimum of agglomeration and in particular with minimal formation of sintered agglomerates.
A method developed to supply fine powders with these characteristics has similarities to the reactor for large particle production but there are also important differences. First, the target particle size of the new reactor is in the size range that the first invention sought to avoid. The seed particles used in the original reactor were in the size range of interest for the present invention but did not meet the criteria for particle morphology or for control of the particle size distribution. This resulted from the relatively large numbers of particles generated in the seed generator--number concentrations that were large enough that appreciable coagulation occurred. The agglomerates thus produced partially sintered, but did not fuse into dense spheres. Coagulation also leads to a relatively broad particle size distribution. To produce uniformly sized nonagglomerated particles requires growing relatively small numbers of very small seed particles primarily by chemical vapor deposition. While as in the original reactor, suppressing the formation of new particles in high enough numbers to compete with the growth of the seeds or to allow significant coagulation.
This was accomplished in the present invention in a single stage reactor in which small numbers of very fine seeds were produced by nucleation of gas phase reaction products. To produce small number concentrations requires that the initial rate of reaction be very slow. The seeds are then grown at a gradually accelerating rate as in the original reactor. What has been demonstrated is that by carrying out a very slow reaction initially, the number concentration of small particles produced in the initial burst of nucleation can be kept small enough to minimize coagulation of the particles during their growth. And secondly, that through careful control of the rate of reaction later in the reaction process suppression of the formation of large numbers of new stable particles (homogeneous nucleation) can be achieved even with the relatively low number concentrations of small particles used in the reactor.